Air circulation is one of the important parameters in a greenhouse. The following text explains how to calculate and select circulating fans by solving a sample example.
Proper air circulation in greenhouses is crucial for maintaining optimal growing conditions for plants. Warm air, being lighter, naturally rises to the top of the greenhouse. This effect is particularly noticeable during colder seasons when heaters are used, leading to uneven temperature distribution across the greenhouse space.
Benefits of air circulation in greenhouses:
Achieves uniform temperature distribution by moving air, even if not all heaters are active.
Promotes better gas exchange on leaf surfaces.
Enhances photosynthesis and CO2 absorption.
Prevents moisture accumulation, reducing fungal diseases.
The Horizontal Air Flow (HAF) system, commonly known as circulation fans among greenhouse operators, plays a key role in achieving these benefits.
Operation of circulation fans: During the operation of fan and pad systems, a uniform airflow moves from the pad to the fan, deactivating the circulation fans. However, in the absence of this system, the circulation fans remain active.
Optimal air circulation speed: The airflow on plant surfaces should be between 0.2 m/s and 1 m/s, with the recommended optimum range being 0.5 to 0.7 m/s. Note that wind speeds exceeding 4.5 m/s can damage plant leaves.
Calculating circulation fan power: The required airflow is determined for a single span. For example, consider a greenhouse with 7 spans, each 9.6 meters wide and 45 meters long (total area: 3,024 m², span area: 423 m²). The airflow requirement per square meter of greenhouse area is 0.01 m³/s.
Calculation:
Vs = 0.01 × 3600 × 423 m³/hr
Vs = 15,228 m³/hr
The 3600 factor converts air volume from cubic meters per second to cubic meters per hour.
Fan selection and spacing: To determine the number and capacity of fans per span, the specifications of circulation fans must be reviewed. As a general rule, the distance between circulation fans should be less than 30 times the fan diameter. Fan specifications provided by manufacturers serve as the basis for selection.
According to the table above, we can use 3 45 fans or 2 60 fans.
In what order are the fans installed?
The first fan is installed 4.6 to 6.1 meters from the beginning of the greenhouse, and the rest will be placed according to the virtual distance mentioned in the table above.
The simplest way to place circulating fans in each opening is to have the even and odd rows facing the same direction, but better efficiency in circulating fans depends on the number of openings in the greenhouse, as shown in Table 8-5.
In single-twin greenhouses, this issue is also addressed by placing reciprocating fans in the tunnel. In single-twin tunnel greenhouses, fans are installed at ¼ and ¾ of the width of the greenhouse, and in multiple-twin greenhouses, they are installed in the center of each opening.
At what height are circulating fans installed?
Circulating fans should be installed 0.5 to 1 meter above the plants.
References:
Greenhouse Management by Paul V. Nelson
Greenhouse Technology and Management by Nicholas Castella
Greenhouse Engineering, published by NRAES
To access files of these books and other greenhouse-related references, you can visit the Telegram channel .
Prepared and organized by: Marsos Cam Greenhouse Industries Hassan Farghani
